The present invention relates to a liquid crystal display and, more particularly, to a thin-film transistor drive type liquid crystal display and a method of fabricating such a liquid crystal display.
A thin-film transistor drive type liquid crystal display (hereinafter referred to as "TFT liquid crystal display") employing thin-film transistors (TFTs) is one known type of liquid crystal display. This known TFT liquid crystal display is provided with TFTs formed on a transparent substrate to control the voltage to be applied to a liquid crystal at each pixel, is capable of displaying sharp images, and is used widely as a display for the terminal equipment of office automation equipment and liquid crystal TV sets.
FIG. 11 is an equivalent circuit of one of the pixels of a conventional TFT liquid crystal display. A TFT 100 is located at the intersection point of a scanning signal line 101 and an image signal line 102 and is connected to a liquid crystal capacitor 103 and a storage capacitor 104. When the TFT 100 is turned on by a signal transmitted through the scanning signal line 101, the potential of the image signal line 102 is written on a pixel electrode, and the liquid crystal capacitor 103 and the storage capacitor 104 are charged. When the TFT is turned off, the liquid crystal capacitor 103 and the storage capacitor 104 hold the charge. However, the resistances of the scanning signal line 101 and the image signal line 102 affect picture quality. Thus, if the resistances of the scanning signal line 101 and the image signal line 102 are large, signals are delayed and arrive at corresponding pixels at different times, and hence sharp images are not displayed. Therefore, it is desirable to form the signal lines of a wiring material having the lowest possible resistivity. An oxide film can be formed on lines of such a wiring material by a well-known anodic oxidation process or the like, and the oxide film can be used as an insulating film for insulating the scanning signal line 101 and the image signal line 102 and as a gate insulating film for the TFT 100. Aluminum is such a preferable wiring material. Wiring lines are formed by forming a film by a vacuum evaporation process, a vacuum sputtering process or the like, patterning a photoresist film formed over the film by a known photolithographic process to form a photoresist mask, and patterning the film by a wet etching process using an etchant or a dry etching process using a chloric gas.
Sometimes, the shape of the self-aligned insulating film formed on the aluminum lines affects the conductive line yield or picture quality adversely. For example, if the shape of the wall of the section of the scanning signal line 101 or the image signal line 102 is an overhanging shape or a nearly vertical shape, the former reduces the conductive line yield of the image signal line 102 and the latter deteriorates the flatness of the protective film, thereby spoiling the orientation of the liquid crystal and deteriorating the picture quality. Moreover, electrostatic focusing due to the edge effect of the gate electrode is liable to cause a dielectric breakdown of the gate insulating film, which deteriorates the characteristics of the TFT. The same problem arises in the gate electrode of the TFT connected to the scanning signal line, the drain electrode connected to the image signal line and the source electrode.
A method proposed to solve such a problem, as disclosed in Japanese Patent Laid-open No. 5-82505, etches an aluminum film with a hydrofluoric acid etchant or a nitric acid etchant to form aluminum wiring lines having a tapered cross section. However, this method damages the glass substrate and the insulating film, and the taper of the cross section of the aluminum wiring lines is excessively small and the aluminum wiring lines have a comparatively large resistance.